Transcript Lecture 4

Lecture 4
Telephone Systems and Cabling
This lecture will cover:
• On-site telephone systems
• Peripheral devices for telephone systems
– voicemail
– call center services
• Copper and fiber that connect telephones
• Computers and networks
Telephone Connections
• The first telephones were wired from one
user directly to another user (1876)
• Later switchboards were installed, operated
by individuals (1877)
• Automatic switching or a central office
switch patented by Almon Strowger (1891)
Switches
• PBX
• Centrex
• Key Systems
PBX and Centrex Systems
• All provide:
– connections between staff in organizations
– connections to the outside world
– the ability for on-site personnel to make calls
within the call territory without paying
telephone usage fees
PBX and Centrex Systems
• Only difference between two systems:
– location of equipment
– ownership of equipment
PBX
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Private Branch Exchange
Located on the customer’s premises
Owned by the user
Eliminates the need to wire each on-site
individual telephone to a central office
• All lines wire to PBX switch, which
connects to a central switch operated by
provider
PBX
• Each employee does not have to pay for:
– telephone line to central office
– telephone calls made between people in the
same office
– dialing within the system can be done with
fewer digits
• 5-1855 Campus Operator
Centrex
• Central Exchange
• Located on the local telephone providers
premises
• Equipment owned by the telephone provider
• Offers same features as PBX in terms of
costs per call, and reduced digit dialing
Key Systems
• Function like a PBX
• Often smaller, but nowadays differences
hard to see
• Formerly had less features than PBX
• User base around 50 to 125 telephones
Connecting Phones to Switches
• Local provider will bring wire into a
building to an interface called rj11cjack
• Most common interface used
• rj21x jack has larger capacity, 25 lines
rj11c
• Location of jack important
• Common point from which to test telephone
lines and trunks
• This is where service begins/ends
– up to jack  telephone provider
responsibility
– from jack to PBX  PBS vendor must test
• demarcation point between provider/user
Features of Centrex
• No-requirement for on-site switching
equipment
• Direct-inward dialing
• Direct-outward dialing
• Automatic identification of dialed calls
Appeal of Centrex
• User
– Save on operator costs, none required
– Detailed billing reports
– Zero on-site space requirements
• Provider
– stimulation of long distance calling
– less use of long distance circuits for call setup
– reduction in staff requirements to place calls
Key Systems
• Loop start system versus ground start (PBX)
• Analog home phones are loop start
• Provides all the functionality of a PBX
Starting a Line
• You “start” a phone line by seizing the
handset
• A supervisory signal is sent to central office
• Two ways to send this signal
– loop start
– ground start
Loop Start
• Most common type of trunk found in
residential installations
• To initiate a call you form a “loop” ring
between your set and the central office
– AC sent to your ringer
– when handset lifted AC interrupted
– DC loop then formed to support the call
Ground Start
• A handshaking routine between CO and
PBX
• Initiated by the PBX to get dial tone
• To see if a trunk is dedicated to the call the
PBX checks to see if the tip lead is
grounded
• Once a trunk is dedicated, the CO will close
the relay and ground the tip lead
Direct Inward Dialing
• Routes calls directly from a local telephone
company to telephone connected to a PBX
or Centrex switch without operator
intervention
• Prior to 80’s expensive feature
• Now commonly sold
• Companies buy entire blocks of phone #’s
Networks
• Send traffic in two main ways:
– circuit-based switching
• Dedicates a path through the network for the entire
duration of the call
– packet-based switching
• Breaks up the message in packets and send packets
over network when available routes become
available, message assembled at final destination
IP Networks for Calls
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Voice calling over IP networks is uneven
Packets share the network
Congestion causes delays
Primarily used within sites and not to
customer
• Low incentive to pursue, long-distance
telephone costs are low
IP Based Phone Systems
• Special phone and routers used
• Equipment digitizes voice traffic and
assembles it into packets
• Compresses the signal so that it takes up
less capacity on the network
IP Telephone Vendors
• Lucent Technologies
• Cisco Systems
• 3Com
Why slow to take off?
• Cost: still too high
• Reliability: local area networks tend to
crash, choppy when network congested
• Existing phone base: works great, not
expensive, lasts about 10 after initial
investment
Add-On Features
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Maintenance contracts
Moves and changes to telephone equipment
Upgrades to the hardware and software
Feature telephones
Wireless phone systems
Peripheral devices: voice mail, automatic
call distribution, call accounting systems
Call Accounting
• Track all phone calls made by users
(SMDR: station message detail recording)
• Allows for charging department for usage
• Indicates amount of traffic on each phone
line
Voice Mail
• Based on a voice store and forward system
• Octel Communications first installed
voicemail in 1980
• PC based systems now available
Automated Attendants
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Programmed statements to answer calls
Or programs that direct calls at busy times
Formerly not a part of voice mail
Voice mail vendors bought out automated
vendors and made it a part of voice mail
Voice Mail Components
• CPU: executes the application programs
• Codecs: convert analog signals to digital
signals and visa versa, compress voice to
decrease storage space
• Software: determines the feature
• I/O cards: to connect phone system with
VM
Voice Mail Components
• Speech recognition cards: specialized cards
and software to recognize spoken
commands such as the numbers 1-10
• Other system components: ports, scanners,
high-speed buses, power supplies, tape and
disk drives
Automatic Call Distribution
• Enable companies to manage incoming calls
and maximize use of employee time
– route calls to agent who’s been idle the longest
– route calls to appropriate agent groups
– if all agents busy, will hold caller in a queue,
route call to another agent, or take voice mail
– provide reports that measure productivity of
agents, trunk usage, number of abandoned calls
Media
• Carry voice and data traffic
• Characteristics of media determine its
speed, accuracy and distance at which
traffic can be carried
– copper
– high-quality copper
– fiber
Copper Cabling
• Electrical properties to copper cabling
create resistance and interference problems
• Signals weaken as they travel
• Resistance of copper slows down signal
• Signals are direct current, very susceptible
to outside electrical sources
• Two copper wires twisted together to limit
interference problems
Twisted Copper
• Used in:
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from set to PBX
from set to key systems
from PCs to wiring closet of a LAN
from homes to nearest local telephone wiring
center
Cat 5 Unshielded Twisted Pair
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Typically installed for voice and data
Carries traffic at 100 Mbps within buildings
Cat 5 or 3 most commonly used
Name indicates that cabling tested and
meats standards set for this type of copper
cabling
Fiber Optics
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Immune to electrical interference
Signals transmitted in the form of light
No electricity present in signal
Highly resistant to taps
Small in size, requires less conduit
Lightweight, no sparking hazard
Enormous bandwidth
Disadvantages Fiber
• Termination and connection costs high
• Needs more care in handling, not flexible
like copper
• Extra electrical power needed
• Specialized technicians needed to install
and test fiber
Fiber Usage
• In all long distance networks
• Between central offices
• From central offices to neighborhood wire
centers
• Internet services provider networks
• Undersea cable runs
• Electric utility networks
Components Fiber System
• Transmitters: convert electrical signals to
light signals
– LED
– Laser: very powerful
• Transducers: convert light signals to
electrical signals
DWDM
• Dense Wavelength Division Multiplexing
• important part of infrastructure for carrying
high-speed voice, data, streaming video and
audio
• Key reason for convergence
• Convergence: applied to high-speed
networks that carry traffic using IP
Advantages DWDM
• Higher capacity over fewer strands of fiber
• Lower costs to upgrade networks, same
fiber used by multiplexing
• Space saving, less amplifying equipment
needed
• Lower ongoing maintenance expenses, less
equipment necessary
Amplifiers and Multiplexers
• Key components of DWDM networks
• amplifiers boost the signal
– every60 miles
• multiplexers combine light from multiple
sources onto a single strand of fiber
– every 360 miles